Cementation methods

ABSTRACT

The present invention provides methods for mineral precipitation and/or cementation of permeable or fractured non-porous materials using isolated urease.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application is a continuation of U.S. Ser. No. 15/803,700 filed Nov. 3, 2017, now U.S. Pat. No. 10,563,233 entitled CEMENTATION METHODS. U.S. Ser. No. 15/803,700 is a continuation-in-part of U.S. Ser. No. 13/944,450 filed Jul. 17, 2013. U.S. Ser. No. 13/944,450 claims priority to and the benefit of U.S. Provisional Patent Application No. 61/672,597 filed Jul. 17, 2012. Each of the foregoing applications are hereby incorporated by reference herein in their entirety.

GOVERNMENT SUPPORT CLAUSE

This invention was made with government support under 0856801 awarded by the National Science Foundation. The government has certain rights in the invention.

SUMMARY OF THE INVENTION

The present invention provides methods for mineral precipitation and/or cementation, comprising combining a column of a permeable starting material or a column of a non-porous but fractured starting material with a mixture comprising

(a) isolated urease;

(b) urea;

(c) a source of divalent cations; and

(d) an organic additive

wherein (a), (b), (c), and (d) are provided in amounts effective and the combining is carried out under conditions suitable to cause carbonate precipitation and/or cementation of the starting material within the column. In one embodiment, the methods are used for one or more of improving bearing capacity of foundations; reducing settlement potential of foundations or embankments; increasing lateral resistance of foundations; enhancing stability of slopes or embankments; reducing lateral earth pressures on retaining walls; increasing passive resistance of retaining walls; increasing capacity of ground anchors or soil nails; increasing the side resistance and tip resistance of deep foundations; facilitating tunneling in running or flowing ground; stabilizing excavations bottoms; soil erosion control; and groundwater control. In another embodiment, the starting material is selected from the group consisting of sand, silt, soil, clay, sediments, sawdust, fractured crystalline rocks, cracked concrete and sedimentary rocks including but not limited to conglomerate, breccia, sandstone, siltstone, shale, limestone, gypsum, and dolostone.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1. Low-Voltage-Scanning electron microscope (LV-SEM) images of (a) Well-grown and cementing calcite crystals; (b) Cementing calcite crystals at inter-particle contact; (c) Indention of quartz surface (central arrows) and nucleation of calcite crystals (arrows on periphery); (d) Calcite crystal growing on quartz surface.

FIG. 2. P-q plot failure envelopes for 20-30 silica sand: ▪Cemented (D_(r)=60%); ∘ Uncemented (D_(r)=60%)

FIG. 3. P-q plot failure envelopes for F-60 silica sand: ▪Cemented (D_(r)=35%); ∘ Uncemented (D_(r)=37%);

FIG. 4. Scanning electron microscope (SEM) images of samples treated using 1 M of urea, 0.67 M of CaCl₂), and 3 g/l of urease enzyme without organic additive (solution 1).

FIG. 5. SEM images using 1 M of urea, 0.67 M of CaCl₂), 3 g/l of urease enzyme, and 4 g/l organic additive (solution 2): (A) CaCO₃ at inter-particle contact points, (B) Inter-particle cementation, (C) Broken CaCO₃ at inter-particle contact.

FIG. 6. Rhombohedral calcite crystals at inter-particle contact points (A). The results of energy dispersive x-ray spectroscopy (EDX) analysis confirm the presence of the elements constituting calcium carbonate and silica (B).

FIG. 7. X-Ray diffraction (XRD) spectrum from powdered XRD analysis showing the corresponding peaks of calcite crystal phase and silica sand. (S: silica sand and C: the calcite).

FIG. 8. SEM images using 0.37 M of urea, 0.25 M of CaCl₂), 0.8 g/l of urease enzyme, and 4 g/l organic additive (solution 3), showing the CaCO₃ cementation at inter-particle contact points (solid-arrow), and broken CaCO₃ at inter-particle contact (dotted line).

FIG. 9. Strength and CaCO₃ values limited to 2.5 MPa and 5% (respectively) to further illustrate unexpectedly high strength values at very low CaCO₃ contents when using the powdered milk organic additive (open triangles) compared to enzyme-induced carbonate precipitation (EICP) without the additive or with a degraded form of the additive. Trend lines added to illustrate strength values of EICP when using the organic additive (open triangles) compared to EICP without the additive or with a degraded form of the additive (all other symbols).

FIG. 10. Soil strength vs. calcium carbonate (CaCO₃) content. Comparison of EICP and microbially-induced carbonate precipitation (MICP) from various studies. UCS=unconfined compressive strength. MICP is microbially mediated process, whereas EICP is an enzyme (non-living) catalyzed reaction.

DETAILED DESCRIPTION OF THE INVENTION

As used herein, the singular forms “a”, “an” and “the” include plural referents unless the context clearly dictates otherwise. “And” as used herein is interchangeably used with “or” unless expressly stated otherwise. All embodiments of any aspect of the invention can be used in combination, unless the context clearly dictates otherwise.

In a first aspect, the present invention provides mineral precipitation and/or host medium cementation methods, comprising combining a permeable starting material or a non-porous but fractured starting material with a mixture comprising

(a) isolated urease;

(b) urea;

(c) a source of divalent cations; and

(d) an organic additive

wherein (a), (b), (c), and (d) are provided in amounts effective and the combining is carried out under conditions suitable to cause carbonate precipitation and/or cementation of the starting material.

In a second aspect, the present invention provides mineral precipitation and/or cementation method for limiting erosion, comprising combining a permeable starting material or a non-porous but fractured starting material with

(a) isolated urease;

(b) urea;

(c) a source of divalent cations;

(d) an organic additive; and

wherein (a), (b), (c) and (d) are provided in amounts effective and the combining is carried out under conditions suitable to cause carbonate precipitation and/or cementation of the starting material and wherein (a), (b), (c) and (d) are combined with the permeable starting material at the surface of the permeable starting material.

As used herein “limiting” refers to at a least 10% reduction in soil erosion compared to soil receiving no erosion treatment or control. Induced carbonate precipitation can enhance the stiffness, strength, and liquefaction resistance of soil. Methods currently under investigation for soil improvement by inducing carbonate precipitation are restricted to medium or coarser sands and are limited by the need to stimulate microbial growth either in the subsurface or ex situ in a reactor vessel and by plugging of the pores by carbonate precipitation and the microbial mass. The methods of the present invention provide significant advantages over prior methods, by permitting precipitation in and/or cementation of much finer permeable materials, such as fine sands and silts, than was previously possible. Furthermore, the methods of the invention mitigate plugging issues that plague prior methods. The methods provide an alternative to commonly used soil improvement techniques such as deep soil mixing, stone columns, penetration and compaction grouting, and rammed aggregate piers. The methods can also be used to fill in cracks in non-porous but fractured or fissured materials, including but not limited to rocks with fractures and cracked concrete.

The methods can be used, for example, in improving the bearing capacity of foundations, reducing settlement potential of foundations and embankments, increasing the lateral resistance of foundations, enhancing the stability of slopes and embankments, reducing lateral earth pressures on retaining walls, increasing the passive resistance of retaining walls, increasing the capacity of ground anchors and soil nails, increasing the side resistance and tip resistance of deep foundations, facilitating tunneling in running or flowing ground (dry or saturated cohesionless soil), stabilizing the bottom of excavations, soil erosion control, groundwater control, and sealing cracks in structural concrete or historical monuments.

“Carbonate cementation” means mineral precipitates that may include one or more cations such as calcium, magnesium, iron and others that may produce one of several phases of carbonate minerals, including but not limited to calcite. In a preferred embodiment, calcium carbonate precipitates form cementation bonds at inter-particle contacts in the permeable starting material. In particular embodiments, the calcium carbonate content of the column is less than 1.5%.

As used herein, “isolated urease” is urease that is purified away from cells and cellular materials. The urease may be synthetically produced or obtained and purified from any suitable source, including but not limited to bacteria, plants, invertebrates, and fungi. Urease enzyme as discussed herein is characterized by the reaction it catalyzes and identified by EC 3.5.1.5 (i.e. Reaction: urea+H₂O=CO₂+2 NH₃). In one embodiment, the urease enzyme is isolated from the jack-bean plant (SEQ ID NO:1). The amino acid sequences of exemplary ureases for use with the present invention are provided below. However, it will be clear to those of skill in the art that any enzyme identified by EC 3.5.1.5 can be used in the methods of the invention, including but not limited to a urease comprising or consisting of any one of SEQ ID NOS: 2-5, where SEQ ID NO:2 is a soybean urease, SEQ ID NO:3 is a Agaricus bisporus urease, SEQ ID NO:4 is a Schizosaccharomyces pombe (strain 972/ATCC 24843) urease, SEQ ID NO:5 is a Sporosarcina pasteurii urease, and SEQ ID NO:6 is a Pseudomonas syringae (strain B728a) urease.

The appropriate amount of urease needed can be determined by one of skill in the art based on the teachings herein; factors to be considered in determining an appropriate amount of urease include, but are not limited to:

(a) urease source type (e.g. Jack bean vs. other source)

(b) urease purity, which dictates enzymatic activity (i.e. rate of conversion of urea to products); and

(c) the stability/half-life of the enzyme matrix used, where the “enzyme matrix” refers to the specific form of the enzyme mixture used such as liquid, powder and/or solid when combined with or used apart from stabilizers, buffers, fillers or other media to facilitate its desired use.

For example, assuming that for practical purposes that transport via diffusion, advection and dispersion is not limiting the availability of urea or calcium to the enzymes—or vice versa—(e.g. we thoroughly mix the soil and cementation constituents or we actively pump the cementation constituents into the soil or do something to assure that the right constituents get to where they need to be), then in a homogenous soil (i.e. without zones of blocked flow or disproportionately high/preferential flow) we could expect an approximately linear relationship between urea conversion and required amount(s) of enzyme needed to convert “x” grams of urea by “y” grams of enzyme over a given time frame. If a soil mass requires a total amount “x” grams of urea to be converted into products for calcium carbonate formation, and “y” grams of enzyme can only convert 50% of “x” during its functional life time, then theoretically twice as much enzyme is needed to fully convert “x” grams of urea.

Urea is an organic compound of the chemical formula CO(NH₂)₂. Urea is a colorless, odorless, highly water soluble substance with very low toxicity (LD50=12 g/kg for mouse, Agrium MSDS), and is widely commercially available. Any suitable source of urea can be used, including but not limited to those disclosed herein.

The appropriate amount of urea can be determined by one of skill in the art based on the teachings herein; factors to be considered in determining an appropriate amount of urea include, but are not limited, to the amount of carbonate required as determined on a stoichiometric basis.

Several divalent cations, primarily alkaline earth metals (including but not limited to calcium and magnesium), that satisfy the crystalline structure constraints of calcite or calcium mineral carbonates can be used to precipitate carbonate minerals in the present methods. Any suitable source of divalent cations can be used, including but not limited to salts of organic and inorganic compounds such as nitrates, nitrites, chlorides, sulfates, oxides, acetates, silicates, oxalates or mixtures thereof.

The appropriate amount of ions can be determined by one of skill in the art based on the teachings herein; factors to be considered in determining an appropriate amount of ions include, but are not limited the required amount of carbonate precipitate as determined on a stoichiometric basis. In one non-limiting example, if 100 grams (approximately 1 mole) of calcium carbonate (CaCO₃) is desired, then 1 mole of urea ((NH₂)₂CO) and 1 mole of calcium (Ca²⁺) are required (the urea also provides the necessary 1 mole of carbon).

Any suitable permeable or non-porous starting material may be used in the methods of the invention, such as those having a particulate structure or those consisting of relatively impervious blocks delineated by an interconnected network of fractures or fissures. In a preferred embodiment, the starting material may be unconsolidated or partially consolidated particulate material such as sand, silt, soil, clay, sediments, sawdust or other material that is amenable to in situ cementation. In further embodiments, the starting material may be sedimentary rocks including but not limited to conglomerate, breccia, sandstone, siltstone, shale, limestone, gypsum, and dolostone. In one preferred embodiment, the starting material comprises sand. In another preferred embodiment, the starting material comprises silt. In a further preferred embodiment, the starting material is fractured crystalline rock or cracked concrete.

The starting material is “permeable” in that it enables sufficient passage of the isolated urease, the urea, and/or the source of calcium or other ions and constituents including, but not limited to, buffers and stabilizers, to enable carbonate precipitation with or without cementation.

The components can be combined in any way suitable in light of the specific starting material, the amount of starting material, the components to be used, etc. In various embodiments, the starting material and components are combined by a technique selected from the group consisting of flushing, injecting, spraying, pouring, dripping or trickling onto or into the starting material. The starting material may also be immersed in one or more ways as described above. In addition, secondary non-specific methods may be employed to facilitate carbonate precipitation including, but not limited to, moisture control measures, crystal seeding, and initiation of nucleation sites. In one embodiment, the methods comprise mixing powdered urease with the permeable starting material prior to percolation of a solution comprising the urea and the divalent ion source.

It will be understood by those of skill in the art that the step of “combining” the permeable starting material with effective amounts of isolated urease, urea, organic additive and ions covers any process that results in the bringing together of the four constituents in a manner that results in precipitation of carbonate minerals and/or cementation in the permeable starting material. The reactants may be added to the starting material simultaneously or sequentially. For example, there may be applications where one or two of the constituents are already present in the permeable starting material, in which case the step of “combining” will involve the addition of only the missing components. In one embodiment, the urea and ions are admixed and then added to the urease prior to application to the permeable starting material. However, it will be appreciated by those of skill in the art that the constituents may be combined in other ways to carry out the method of the invention.

By manipulating the relative effective amounts of the various components, the methods of the present invention enable the user to control carbonate precipitation and cementation by controlling the amount of carbonate formed and the rate at which it is formed. This flexibility means the methods of the present invention can be used in a wide range of applications from those that require a reasonably modest strength increase in the starting material to those that require larger increases.

The effective amounts of the various reactants combined according to the method of the present invention may vary depending, at least, on the amount of urease used, the characteristics of the permeable starting material and the conditions under which precipitation and/or cementation is to occur, the desired final strength, stiffness, or permeability of the treated porous material and the amounts of the other reactants in the reaction mix. The present application enables those of skill in the art to determine the relative amounts of the various reactants required for a given application and to apply the method to various starting materials and for a variety of end uses. The method of the present invention may be adapted to allow for the rate of mineral precipitation and/or cement formation to be controlled, as required. When rapid or slow formation of the precipitate or cement is desired the amounts and/or relative amounts of the reagents can be selected accordingly to bring about the desired rate of formation. In one non-limiting example, enhancement of the methods may comprise providing stronger nucleation sites on particles of the permeable starting material by high-pH pretreatment of the permeable starting material to be improved.

Depending on the requirements of a particular application or mode of use of the present invention, rapid formation of the precipitate and/or cement may be required. Alternatively it may be preferred for the precipitate or cement to be formed slowly. Based on the teachings herein, those of skill in the art will be able to modify the protocol to attain faster or slower formation of the precipitate and/or cement.

The methods of the invention may be performed a single time or may be repeated (once, twice, three times, or more) in order to attain the desired amount of cementation strength, stiffness increase, or permeability reduction. When the method is repeated to gain incremental increases in strength or stiffness or reduction in permeability, not all of the reagents need to be added each time. For example, residual urease activity may still be sufficient for one or more subsequent rounds of the method. A skilled person is readily able to determine the particular amounts of reagents required for use in subsequent rounds of the method of the present invention.

The methods may be applied in situ without disturbing the permeable starting material. This is particularly important for applications where the permeable starting material is delicate or fragile or for other reasons must not be disturbed. For example, when applied in the field where the soil to be improved (e.g. made resistant to earthquake-induced liquefaction) is underneath an existing structure or facility that is sensitive to ground movement (e.g. settlement or heave). As will be understood by those of skill in the art, the methods may comprise use of other components as appropriate for a given use. In one embodiment, the methods may further comprise use of an “organic additive.” to increase enzyme stability, functional time and focus mineral precipitation. As used herein “organic additive” refers to a substance having reaction dependent biochemical properties such that the additive responds to changes in the chemical environment (i.e., ion concentration and ion type) by changing it properties (i.e., solubility, structural conformation, aggregation, and/or surface charge). The organic additives disclosed herein are water soluble and comprise protein stabilizing properties. In particular embodiments, the organic additive provides nucleation points in the permeable starting material. In some embodiments, the organic additive is powdered milk. In other embodiments, the organic additive is a colloid which is a homogeneous, noncrystalline substance consisting of large molecules or ultramicroscopic particles of one substance dispersed through a second substance. Colloids include gels, sols, and emulsions; the particles do not settle and cannot be separated out by ordinary filtering or centrifuging like those in a suspension), and hydrogels. In some embodiments the organic additive is present a concentration of about 4 g/L in a mixture comprising the isolated urease, urea and source of divalent cations. As used herein “about” includes concentrations within +/−5% of the recited concentration. In various further embodiments, the organic additive is present a concentration of 1-10 g/L. In various further embodiments the organic additive is present a concentration of 3-5 g/L. In various further embodiments the organic additive is present a concentration of 1 g/L, 2 g/L, 3 g/L, 5 g/L, 6 g/L or 7 g/L in a mixture comprising the isolated urease, urea and source of divalent cations. The methods of the present invention provide significant advantages over prior methods by providing significantly higher strengths at significantly lower carbonate content through focused mineral precipitation. This low carbonate content requires only one cycle of treatment, providing further benefits over previous treatment schemes for soil improvement, which typically require multiple cycles of treatment. The methods can be carried out under any temperature conditions suitable to promote carbonate cementation.

In a preferred embodiment, the permeable starting material comprises a column of the permeable starting material. As used herein, a “column” refers to relatively linear prisms of stiffened and/or strengthened soils used to reinforce the uncemented soil mass and/or transfer load to greater depths in the soil stratum. As will be understood by those of skill in the art, the prism of soil extends below a surface of the starting material. In various embodiments, the column is at least 0.5 meters long (deep) and 0.1 meters in diameter. In various further embodiments, the column is at least 1, 2, 3, 4, 5, 6, 7, 8, 9, 10, 15, 20, 25, 50, or more meters long/deep. In various further embodiments, the column is at least 0.2, 0.3, 0.4, 0.5, 0.6, 0.7, 0.8, 0.9, 1, 1.5, 2, 2.5, 3, 3.5, 4, 4.5, 5, 7.5, 10, or more meters in diameter. In this embodiment, the methods comprise combining the starting materials so that carbonate precipitation and/or cementation of the starting material occur at a desired location in the column. In one embodiment, carbonate precipitation and/or cementation occurs at only one or more specific areas within the column, such as at a specific location where the starting materials are combined within the column. In another embodiment, carbonate precipitation and/or cementation occur throughout the column.

This embodiment provides further improvements over prior methods, which are focused on improvement of the entire mass of soil in the improvement zone. In this embodiment, the methods permit production of stone-like materials via local cementation. Columns of improved starting materials, such as soils, have broad application in geotechnical practice, including improvement of foundation bearing capacity, support of embankments, slope stabilization, stabilization of the base of excavations, support of underground openings, and a variety of other geotechnical purposes. The limitations associated with bio-plugging can also be mitigated by using this technique to improve columns of soil (rather than the entire soil mass).

In one non-limiting example, consider a 4.2-meter long, 1-meter diameter column of soil with a porosity of 0.31 is to be improved by precipitating 5% calcium carbonate by weight. This column has a total volume of 3.29 m³ and a pore space of 1.0 m³. Assuming a calcium carbonate density of 2.71 g/cm³ (2710 kg/m³), approximately 136 kg of CaCO₃ is used to precipitate in the improvement zone (i.e. the column). To obtain 136 kg of CaCO₃ (1,360 moles), 81.6 kg of urea (1,360 moles) and 54.4 kg of calcium (1,360 moles) are used. Since there are 2 amine groups per urea molecule, 2,720 moles of ammonia nitrogen and 1,360 moles of carbon dioxide are released from 81.6 kg of urea. Assuming a minimum urease activity of 15,000 units/gram urease powder (type 3 urease from Jack bean [Sigma Aldrich]), where one-unit is the release of 1.0 mg ammonia nitrogen from urea in 5 minutes at pH 7.0 at 20° C., then 10 grams of this particular grade of enzyme (150,000 units) will release 150 grams of ammonia nitrogen 0.8 moles) in 5 minutes. Therefore, 26 hours (1546 minutes) are required to fully catalyze the release of carbon dioxide and ammonia from 81.6 kg of urea.

In a second aspect, the present invention provides kits comprising

(a) isolated urease;

(b) urea;

(c) a source of divalent cations, preferably calcium ions; and

(d) an organic additive

wherein (a), (b), (c), and (d) are provided in amounts effective to cause carbonate cementation when combined with a permeable starting material.

All definitions, embodiments, and combinations thereof of the first aspect apply equally to this second embodiment, unless the context clearly dictates otherwise. Thus, the kits may further contain any of the components or combinations thereof disclosed for use with the methods of the invention, including but not limited to stabilizers, buffers, nucleation aids, etc.

The use of plant-derived urease enzyme offers many benefits over the use of microbially-generated urease to induce carbonate cementation for soil improvement, a process that has garnered much attention recently. In this biogeochemical soil improvement process, urea hydrolysis is catalyzed by the urease enzyme (urea amidohydrolase), a widely occurring protein found in many microorganisms, higher order plants, and some invertebrates, to precipitate calcium carbonate in the presence of calcium. The calcium carbonate precipitate (CaCO₃) forms cementation bonds at inter-particle contacts and also fills the void space in granular soils. Urease is a nickel-dependent, metalloenzyme that is approximately 12 nm by 12 nm (subunit). By comparison, nearly all known bacteria that generate urease are greater than 300 nm in diameter, with the majority in the range of 500-5000 nm. As soil improvement by ureolytic carbonate precipitation requires penetration of the pore spaces by the improvement media, the small size of the enzyme affords the use of plant-derived enzyme a distinct advantage over microbial methods, including the ability to penetrate finer grained soils and less sensitivity to bioplugging (clogging of the pore space by the precipitate). An additional benefit of the use of plant-derived enzyme compared to microbially-derived enzyme is that the 100% of the carbon in the substrate is available for conversion to CaCO₃. Furthermore, plant derived enzyme is widely available.

EXAMPLES Example 1: Carbonate Cementation Via Plant Derived Urease

Methods

Ottawa 20-30 Sand

Laboratory column tests were conducted using plant derived urease to induce CaCO₃ precipitation in Ottawa 20-30 sand. These tests were carried out in 6″×2″ (152 mm×51 mm) acrylic tubes and membrane-lined 2.8″×6″ (71 mm×152 mm) split molds (for creating specimens for triaxial testing). Three acrylic tubes and two columns for triaxial testing were filled with 20-30 Ottawa silica sand (mean grain size 0.6 mm, coefficient of uniformity 1.1) and treated as follows: tube #1: the sand was dry pluviated via funnel at ≈3″ (76 mm) drop height and then received 5 applications of a cementation solution containing urea and calcium chloride mixed with 1.4 g/L enzyme (total solution volume≈300 ml); tube #2: sand was added in same manner as tube #1 and then received 2 applications (≈150 ml total) of the same cementation solution mixed with 1.4 g/L enzyme; tube #3: the lower-third of tube was filled with sand and dry enzyme (≈3 g), the remainder of the tube contained dry pluviated sand without enzyme, and the tube then received 2 applications (≈150 ml) of the cementation fluid with no enzyme added.

Approximately 100 mL of a pH=7.8 solution containing 383 mM urea (reagent grade, Sigma-Aldrich), 272 mM CaCl₂-2H₂O (laboratory grade, Alfa Aesar) was used for the first application in each acrylic tube. Subsequent applications employed approximately 50 mL of a pH=7.6 solution containing 416 mM urea and 289 mM CaCl₂-2H₂O. Solution concentrations, while variable, were formulated within a reasonably similar range as a matter of convenience. In each application, the cementation fluid was poured into the top of the acrylic tube with the bottom closed off. The cementation fluid was allowed to stand, loosely covered, in the acrylic tube for at least 24 hours and then drained out the bottom of the cylinder. The next application followed immediately after drainage was complete. Drainage was accomplished by puncturing the base of the cylinder with a 20-gauge needle. When drainage was complete, the needle was removed and the puncture was plugged with a dab of silicone. Occasionally, the needle became plugged and an additional needle was inserted through the base. The triaxial columns were filled with sand in the same manner as tube 1 and then received 2 applications (each application ≈250 ml) of cementation solution with 1.4 g/L enzyme.

In each application of cementation fluid, the fluid was added until it rose to approximately ½-inch (12-mm) above the soil line. After 2 applications, tubes #2 and #3 were allowed to air dry for several days and then analyzed. Experimentation with tube #1 was continued for several more days as three more batches of cementation fluid were applied. The last 2 applications of cementation fluid were allowed to slowly drain through the needle in the base immediately after application rather than sit for 24 hours (drainage rate≈10-25 ml/hour). The triaxial columns were allowed to stand for at least a week after the second cementation fluid application and then drained.

After drainage was complete, the triaxial columns were moved to a triaxial testing device. After draining the specimens from the acrylic tubes and after the completion of the triaxial tests, all samples were triple washed with de-ionized water. Tubes #2 and #3 were separated in 3 layers, while tube #1 was separated into six layers (for better resolution). Each layer from the specimens in the acrylic tubes and the entire mass of the triaxial specimens were acid washed to determine CaCO₃ content by oven drying for 48 hours, weighing, digesting with warm 1M HCl, washing, drying, and reweighing to determine carbonate mineral content.

Several of the cemented specimens were analyzed for mineral identification using X-Ray Diffraction (XRD). Samples were ground in an agate mortar and pestle and powdered onto a standard glass slide for analysis. Scanning electron microscopy (SEM) imaging was performed on intact cemented chunks of material with an Agilent 8500 Low-Voltage SEM (LV-SEM). A LV-SEM is a field emission scanning electron microscope capable of imaging insulating materials, such as organic and biological substances without the need for a metal coating and without causing radiation damage to samples.

Ottawa F-60 Sand

A triaxial column was prepared using Ottawa F-60 silica sand (mean grain size 0.275 mm, coefficient of uniformity 1.74) to investigate enzymatic ureolytic CaCO₃ precipitation in a finer grained material. The specimen was prepared in the same manner as described for the triaxial columns for the Ottawa 20-30 sand. The cementation fluid for the first of the two applications contained approximately 2.0 g/L enzyme, 400 mM urea (reagent grade, Sigma-Aldrich), 300 mM CaCl₂-2H₂O (laboratory grade, BDH) at pH=7.7. The fluid for the second application contained 1 M urea-CaCl₂₋₂H₂O solution at pH=7.8 without any enzyme. After the test, the triaxial specimen was washed and subject to acid digestion in the same manner as the Ottawa 20-30 triaxial specimens.

Results

Acrylic Tubes

Approximately 100 ml of cementation solution was delivered per application for the first application in each acrylic tube. However, the amount of solution the tube would accept was notably reduced in subsequent applications, when less than 75 ml was typically required to fill the tubes to ≈½ inch (12 mm) above soil line. At the conclusion of the experiment, precipitation was visible along the entire length of tubes 1 and 2. Internally the cementation was variable, with some highly cemented zones and other zones with little to no cementation.

Tube 1 yielded mostly small, loose chunks of sand with strong effervescence upon digestion. Most of this column appeared un-cemented and exhibited unusually viscous behavior when wet. A fairly large (compared to column diameter) piece of strongly cemented sand (not breakable without tools) formed in the deepest layer of tube 1. Tube 2 had many small chunks of weakly cemented sand with strong effervescence upon digestion. Tube 3 had little to no precipitation in the top layer (i.e. this layer did not show any indication of carbonate upon acid digestion.) The deepest layer of tube 3 contained many pieces of weakly cemented sand that effervesced strongly upon digestion. The middle layer of tube 3 contained a few pieces of cemented sand that effervesced moderately upon digestion. The results from the acid washing are presented in Table 1.

TABLE 1 Results from Experiment Set 1 using 20-30 Ottawa silica sand Summary of Results Weight Amt. Total Theor. Change of Amt. Max via CaCO₃ CaCO₃ CaCO₃ Tube # Layer Digestion (g) (g) (g) 1 1  11% 3.57 11.8 ≈14.5 2 3.8% 1.67 3 2.7% 1.73 4 2.1% 1.40 5 2.3% 1.74 6 2.0% 1.64 2 1 0.76%  0.63 2.07 ≈4.35 2 0.65%  0.69 3 0.49%  0.75 3 1 0.23%  0.31 3.57 ≈4.35 2 0.58%  0.63 3 1.7% 2.63

The theoretical maximum CaCO₃ content is the stoichiometric maximum balanced on initial concentrations. The primary experimental differences between the tests are (1) the number of applications of cementation fluid and (2) the manner in which the urease was delivered. The results indicate that there is greater carbonate precipitation with increasing number of applications, as expected. The data show more precipitation in (or on) the top layer of tubes 1 and 2 but not in tube 3, as the enzyme was physically confined to the lower-third layer in tube 3 during sample preparation. In the top layer of tube 3, where no urease was mixed with the sand, carbonate precipitation was nearly undetectable. There was no visual evidence of precipitation and practically no measurable change in weight of this layer after acidification (weight change=0.23%). In the bottom layer of tube 3, where 3 g of dry enzyme was mixed with the soil, there was a weight change of 1.7% following acid washing. The middle layer of this specimen had a minor change in weight (0.58%), possibly due to uneven distribution of the layers during preparation or splitting of the specimen or to upward migration of urease from the bottom layer.

XRD analysis confirms that calcite is the mineral phase present in the cemented soil chunks. LV-SEM images, presented in FIG. 1, show silica (quartz) sand particles cemented with calcium carbonate and various morphological features associated with the cementation process on the silica surface.

Triaxial Columns

The three triaxial sand columns (2 Ottawa 20-30 sand columns and 1 Ottawa F-60 sand column) were tested in drained triaxial compression prior to acid digestion. All three columns were able to stand upright after removal of the split mold. The results of the triaxial compression tests performed on the 20-30 Ottawa sand are presented in FIG. 2 and the results for the F-60 Ottawa sand are presented in FIG. 3. The carbonate cement content for one of the 20-30 silica sand columns was 2.0% CaCO₃ (by weight). The carbonate content of the other 20-30 Ottawa sand column could not be quantified due to unintended sample loss. The carbonate cement content for the finer grained F-60 Ottawa sand was 1.6% CaCO₃ (by weight). The results show substantial strength increase for all 3 sand columns tested.

CONCLUSION

Sand column tests have shown that agriculturally-derived urease can be used to induce calcium carbonate precipitation in sand. Sand columns were developed using Ottawa 20-30 and F-60 sand and three different preparation methods: dry pluviation followed by percolation of a calcium-urease-urea cementation solution, pluviation into a calcium-urease-urea cementation solution, and mixing the sand with urease prior to pluviation with a calcium-urea solution. Cementation was observed in all of the columns. XRD and SEM testing confirmed that calcium carbonate (specifically calcite) was the cementing agent. Acid digestion showed that increased applications yielded correspondingly greater carbonate precipitation. The quality of cementation, as determined by the effort needed to break apart cemented chunks of sand, varied depending on the sampling location within the column. Triaxial test results on cemented columns showed substantial strength increase over non-cemented columns at the same relative density.

Example 2: Enzyme Induced Carbonate Precipitation with a Powdered Milk Organic Additive

Methods

EICP Treatment Solutions

EICP solutions were prepared by dissolving EICP constituents in deionized water with an organic additive (test) and without the organic additive (control). The experiments employed two different concentrations of the substrate (urea) and enzyme in the organic additive-enhanced solution: one a relatively low concentration (0.37 M urea, 0.25 M CaCl₂), 0.8 g/l urease) and one a relatively high concentration (1 M urea, 0.67 M CaCl₂), 3.0 g/l urease). Only the solution with the higher concentrations was used for the control specimens. Reagent grade (≥99% purity) urea and calcium chloride in the form of CaCl₂-2H₂O were used, and the enzyme used was jack bean urease from Fisher Scientific. Concentrations of constituents used in the tests were:

Solution 1: 1 M of urea, 0.67 M of CaCl₂), and 3 g/l of urease enzyme without organic additive.

Solution 2: 1 M of urea, 0.67 M of CaCl₂), 3 g/l of urease enzyme, and 4 g/l powdered milk organic additive.

Solution 3: 0.37 M of urea, 0.25 M of CaCl₂), 0.8 g/l of urease enzyme, and 4 g/l powdered milk organic additive.

Sample Preparation

Soil columns were prepared in acrylic cylinders that were filled with Ottawa 20-30 sand (US Silica Company), a uniform quartz sand with a mean grain size of 0.6 mm, to investigate enzymatic ureolytic CaCO₃ precipitation. The acrylic cylinders measured 101.6 mm high and 50.8 mm in inner diameter (4″×2″). A polypropylene (PP) liner was placed inside of the acrylic columns to ease sample extraction for UCS testing. The bottoms of the columns were closed with rubber caps and sealed with silicone glue to minimize leakage. The PP liner extended above the acrylic cylinder and its top was loosely closed to minimize evaporation of the EICP solution. Each soil column was prepared by first pouring 75 ml of the EICP solution (corresponding to slightly more than one pore volume of the final specimen) into a bowl containing 350 g of 20-30 Ottawa sand and then thoroughly mixing the sand and solution. The sand-EICP solution mixture was then quickly placed into the acrylic column in three layers. Each layer was compacted by tamping 25 times with a small hammer to reach a relative density of 90% (based upon the dry weight of soil). The hammer weighed 11.56 N (2.6 lb.). After compaction, the solution was always a few millimeters above the soil, indicating that the soil is under an inundated (almost saturated) condition.

Sample Curing

The compacted soil and cementation fluid was allowed to stand in the acrylic cylinders for at least 72 hours at room temperature. Any fluid remaining in the columns was then drained out from the bottom of the cylinder, followed by an 80 ml of DI water rinse (slightly more than one-pore volume) that was added slowly to the top of each column to flush out any remaining EICP solution and soluble salts. After draining and flushing were complete, each treated specimen was extracted from the acrylic cylinder. Then, selected samples were soaked in DI water for at least one half hour (as described subsequently). All samples were placed into an oven at (40° C.) until a constant mass was achieved. Once oven drying was complete, specimens were prepared for the UCS Test.

Unconfined Compressive Strength Testing (UCS Testing)

UCS testing was conducted in according with ASTM D4918. Prior to performing the UCS test, the top and bottom surface of each sample was leveled using plastic steel putty from ITW Devcon.

Triaxial Testing

Undrained triaxial tests were conducted on samples prepared using Solution 2 in order to measure the shear strength of the treated soil. In these tests, the Ottawa 20-30 sand was prepared at a relative density of 40%, instead of 90% for comparison to results of specimens compacted to 40% relative density and treated using microbially induced carbonate preparation (MICP). The samples were prepared by pouring the dry soil through a funnel (air pluviation of dry soil). Before applying the confining pressure, each sample was back pressure saturated until Skempton's B parameter was above 0.95. Samples were tested at confining pressures of 50 kPa, 100 kPa, and 150 kPa. Each test was performed at a constant axial strain rate of 0.5% per minute to a final strain of 20%.

Acid Digestion

Following UCS and triaxial testing, intact pieces of samples were dried in an oven at 105° C. for 24 hours, weighed, and then washed with a strong (4 M) hydrochloric acid (HCl) solution to determine the mass of calcium carbonate precipitates. Following treatment with HCl, the samples were rinsed with deionized (DI) water and dried in the oven at 105° C. The mass difference before and after the acid-wash was considered as the mass of the carbonates precipitated in the specimen.

Micro Scale Identification Analysis

X-Ray Diffraction (XRD) analysis was performed on intact pieces of selected specimens to identify mineral crystal phases existing in each sample. The samples were ground using an agate mortar and pestle and powdered onto a standard glass slide for XRD analysis. Scanning electron microscopy (SEM) imaging was also performed on intact cemented chunks of material. Energy dispersive X-ray (EDX) analysis was also carried out in conjunction with SEM imaging to determine the elemental composition of each sample. The samples were coated with carbon prior to SEM/EDX analysis.

Results

Tests Using EICP Solution 1

Two columns were prepared using EICP Solution 1, i.e., composed of 1 M urea, 0.67 M CaCl₂, and 3 g/L of urease enzyme without organic additive. The results of UCS testing and acid digestion on these two specimens are shown in Table 2.

TABLE 2 Results of UCS and Carbonate Precipitation Using Solution 1 Column CaCO₃ (%) Peak Strength (kPa) 5-1 1.63 133 5-2 0.98 158

SEM images were taken of these samples to provide visual evidence of CaCO₃ precipitation. The image in FIG. 4 shows that the calcium carbonate precipitate was spread around the surface of the sand particle.

Tests Using EICP Solution 2

A series of specimens were prepared using EICP Solution 2, a solution containing 1 M of urea, 0.67 M of CaCl₂, 3 g/l of urease enzyme, and 4 g/l of powdered milk organic additive; this solution was the same as Solution 1 except with the addition of organic additive. UCS testing of specimens treated with Solution 2 yielded significantly higher strength than those prepared with Solution 1 (i.e., by a factor between 6 and 20) at similar carbonate content. The UCS and carbonate content of the tests using Solution 2 are presented in Table 3.

TABLE 3 Strength and percentage of precipitation mass for samples treated with Solution 2 1M Urea, 0.67M CaCl₂, 3 g/L of urease, 4 g/L powdered milk Peak CaCO₃ Column Strength (kPa) (%) Rinsed or Soaked 5-3  911 1.02 Soaked Overnight 5-4 1817 0.82 Rinsed in Column 5-5 1010 0.78 Rinsed in Column 5-6   463* 0.84 Rinsed in Column 5-7 1654 0.79 Rinsed in Column 5-8 1763 1.17 Rinsed in Column *Cap failed, so tested just the bottom of the column Scanning electron microscpe (SEM) images of specimens prepared using Solution 2 are presented in FIG. 5. These images exhibit a pattern of precipitation that may be characterized as densely agglomerated calcite crystals at inter-particle contact points. FIG. 6 presents the results of EDX analysis focused at the contact point of a specimen treated using Solution 2. The EDX analysis yields a carbonate percentage of around 2%, higher than measured for the bulk sample (as expected), but still significantly lower than reported by others for the same UCS. The results from XRD analysis presented in FIG. 6 demonstrate that the precipitate is primarily calcite. Tests Using EICP Solution 3

Two samples were treated using Solution 3, a solution with a lower concentration of constituents than Solution 2. Solution 3 contained 0.375 M urea, 0.25 M CaCl₂, 0.8 g/L enzyme, and 4 g/L of powdered milk organic additive (37.5% of the urea and calcium in the other solutions). The results of the tests on these specimens, shown in Table 4, still yield a strength of over 1 MPa at very low CaCO₃ content. Furthermore, the amount of CaCO₃ precipitated was around the theoretical maximum.

TABLE 4 Results of UCS and Carbonate Precipitation Using Solution 3 0.375M Urea, 0.25M CaCl2, 0.85 g/L of urease, 4 g/L powdered milk Column Peak Strength (kPa) CaCO₃ % Rinsed or Soaked 5-26 1000 0.57 Soaked overnight 5-27 1396 0.71 Rinsed in Column SEM images of the specimens prepared using Solution 3, the lower concentration of urea, CaCl₂), and urease enzyme, are shown in FIG. 8. These images shows the same pattern of concentrated precipitation at inter-particle contacts than seen when the higher concentration of urea, CaCl₂), and urease enzyme was used with organic additive. Comparison to Previous EICP Test Results

FIG. 9 compares the results of UCS tests on soil improved using EICP reported by others in the literature to the results reported herein. Previous testing reported in the literature by Yasuhara at al., Soils and Foundations (2012), 52(3):539-49, the only other group to report in the literature on strength testing of EICP-improved soil, yield strengths on the order of 100 KPa (0.1 MPa) at carbonate content of around 1 percent and a strength of around 400 KPa at carbonate content of around 4.6%. Previous testing using the control EICP solution conducted by Hamdan (2015), “Applications of Enzyme Induced Carbonate Precipitation (EICP) for Soil Improvement,” Doctoral Dissertation, Arizona State University, and by Almajed et. al (2017) “Enzyme Induced Carbonate Precipitation (EICP) for Soil Improvement,” Doctoral Dissertation, Arizona State University, using a degraded organic additive-enhanced EICP solution (a solution prepared using expired powdered milk) provided results similar to those reported by Yasuhara at al., Soils and Foundations (2012), 52(3):539-49. As shown in FIG. 9, the tests conducted using the fresh organic additive surprisingly yielded significantly higher strength at lower carbonate content and with just one cycle of treatment.

Comparison to Previous MICP Test Results

FIG. 10 compares the UCS test results reported herein to both the tests on EICP-improved soil conducted by others (those reported above and additional testing conducted by Gomez and Dejong, “Engineering Properties of Bio-cementation Improved Sandy Soil,” Conference Proc. from Grouting 2017, (2017), 23-33 and van Paassen et al., “Quantifying Biomediated Ground Improvement by Ureolysis: Large-Scale Biogrout Experiment,” J. Geotechical & Geoenvironmental Engineering (2010): 1721-28) using multiple treatment cycles and the results of tests on soil improved by MICP. It can be seen that the tests conducted using the organic additive-enhanced EICP solution yield significantly higher strengths at low carbonate content (carbonate content less than 1.5% and that multiple cycles of treatment and a significantly higher carbonate content (greater than 12%) was required to achieve strengths comparable to those achieved with the enhanced EICP solution. 

What is claimed is:
 1. A cementation method, comprising combining a column of a permeable starting material or a column of a non-porous but fractured starting material with a mixture comprising: (a) isolated urease; (b) urea; (c) a source of divalent cations; and (d) an organic additive, wherein (a), (b), (c) and (d) are provided in amounts effective and the combining is carried out under conditions suitable to cause carbonate precipitation and/or cementation of the starting material within the column; and wherein the combining comprises injecting into the column of starting material, under pressure, a solution comprising the urea and the source of divalent cations.
 2. The method of claim 1, wherein the method is used for one or more of improving bearing capacity of foundations; reducing settlement potential of foundations or embankments; increasing lateral resistance of foundations; enhancing stability of slopes or embankments; reducing lateral earth pressures on retaining walls; or increasing passive resistance of retaining walls.
 3. The method of claim 1, wherein the starting material comprises one or more of sand, silt, soil, clay, sediments, sawdust, fractured crystalline rocks, cracked concrete, or sedimentary rocks.
 4. The method of claim 1, wherein the desired location is a desired depth combining occurs only below a top surface of the column.
 5. The method of claim 1, further comprising performing a high-pH treatment of the column of the permeable starting material or the column of the non-porous but fractured starting material before the combining.
 6. The method of claim 5, wherein performing the high-pH treatment comprises initiating nucleation sites.
 7. The method of claim 1, wherein the combining step is carried out without disturbing or mixing the permeable starting material or the non-porous but fractured starting material.
 8. The method of claim 1, wherein the combining comprises mixing the urease with the starting material.
 9. The method of claim 8, wherein the injecting is performed via an injection tube.
 10. The method of claim 8, wherein the urease is mixed with starting material in only a portion of the column.
 11. The method of claim 1, wherein the organic additive is a homogenous, noncrystalline colloid.
 12. The method of claim 1, wherein the organic additive is powdered milk.
 13. The method of claim 1, wherein the organic additive is present in the mixture at a concentration of 1-10 g/L.
 14. The method of claim 12, wherein the organic additive is present in the mixture at a concentration of 3-5 g/L.
 15. The method of claim 12, wherein the organic additive is present in the mixture at a concentration of about 4 g/L. 